Electrical and mechanical equipment often requires cooling while in operation. Both liquid (e.g. oil) and gas (e.g. air) cooling schemes are well known. Air cooling schemes may be either active (e.g. fan driven) or passive (i.e. rely on an existing pressure gradient to introduce a flow of cooling air to the equipment). In gas turbine engine applications employing a passive air cooling scheme to cool the on-board generator(s), a minimum amount of airflow is required to cool the generator. A pipe that is attached to the bypass duct on the gas turbine engine has been sized to allow this minimum mass flow of air when the air pressure difference between the bypass duct and the atmosphere is very small, for example approximately 2 lbs. per square inch differential (PSID). This is the condition at idle of the gas turbine engine when the aircraft is on the ground. When the engine speed is increased for takeoff or flight conditions, the pressure difference between the bypass duct and atmosphere is increased to approximately 10 PSID. This causes more cooling air than is required to cool the generator, to flow through the pipe. The resulting oversupply of cooling air increases the Specific Fuel Consumption (SFC) of the gas turbine engine. Therefore it is desirable to use a valve to control the cooling airflow through the pipe between the bypass duct and the generator.
Fugii describes an automatic airflow smoothing valve in his U.S. Pat. No. 3,718,516 issued on Feb. 27, 1973, for ensuring a constant output flow in spite of great changes of the pressure of the input airflow. Fugii's valve includes a hollow, elongated open-ended casing, and a rod is mounted in the upper end of the casing and depends therealong. A primary coiled extension spring is mounted on the rod and a pair of secondary coiled extension springs are attached at their upper ends to the primary coiled extension spring to form a star connection. A pair of flapper disc halves hinged together are pivotally mounted within the casing at the lower end thereof. The lower ends of the secondary coiled extension springs diverge from the primary coiled extension spring and are attached to the respective flapper disc halves. Perforations are provided in the flapper disc halves to permit airflow when the valve is closed against the spring forces by the input airflow under relatively high pressure. When the pressure of the input airflow is reduced, the extended springs cause the valve to open to an extent corresponding to the pressure of the input airflow, in order to ensure the constant output airflow.
Smirra describes a hinge valve in his U.S. Pat. No. 3,559,679, issued on Feb. 2, 1971 for controlling fluid flow in a conduit. Smirra's valve comprises a support member extending diametrically across the interior of the conduit. Two flap members pivotally mounted to the support member are adapted to move from an open position allowing flow of fluid through the conduit to a closed position preventing flow of fluid. The flap members are actuated by a piston that is pivotally attached to linkage connecting the flap members and moves within a hydraulic cylinder under fluid pressure or spring force. Both Fugii's and Smirra's valves are complicated and include more moving parts than the two flap members, which compromises the reliability of their valves. Both Fugii's and Smirra's valves include coiled extension or compression springs positioned axially within the casing or the conduit, which cause the corresponding parts to move axially such that the valves cannot be made compact in the axial dimension.
The cooling system of an aircraft gas turbine engine requires a valve that does not necessarily maintain a constant output volume of airflow, but must be very reliable in performance, and compact in size to reduce the weight thereof. Therefore, there is a need to develop an improved valve to be used in the cooling system of aircraft gas turbine engines in order to control the cooling airflow to the generator.